It has been 60 years since the invention of contact lenses in early 1950s. The earliest contact lenses were made of firm materials such as polymethyl methacrylate (PMMA); due to the hard texture and poor oxygen permeability and hydrophilicity of the firm materials, the early contact lenses had short wearable time and tended to cause strong discomfort or foreign body sensation. The invention of soft contact lenses in the 70s was revolutionary. The soft contact lenses utilized a hydrogel mainly consisting of 2-hydroxy ethyl methacrylate (HEMA). Comfort of wearing contact lenses had largely improved due to their high water absorbability and the capability to soften and retain high water content upon hydration. However, the soft contact lenses could only be worn for 8-12 hours due to their poor oxygen permeability; long-term wearing had commonly led to oxygen-deficient corneal edema and angiogenesis.
Hard contact lenses with ultra high oxygen permeability entered the market in early 80s. However, hard materials causing foreign body sensation to the wearers remained an unsolved issue, thus leading to low consumer acceptance. Accordingly, there has been a need for contact lenses having high oxygen permeability, excellent comfort and long wearable hours. As the contact lenses industry continued to develop, highly oxygen permeable soft silicones or silicone hydrogels have eventually became the mainstream.
Silicone is an unstable hydrophobic material; consequently, in spite of high oxygen permeability, contact lenses made of silicone contain large amount of hydrophobic dry spots on the surfaces, again causing discomfort to the eyes of the wearers. To improve wearability of silicone contact lenses, the industry has came out with treating the surface of silicone or silicone hydrogel with plasma for conjugation with hydrophilic materials, such as HEMA, glycerol methacrylate (GMA), glycerol methyl methacrylate (GMMA), N-vinyl pyrrolidone (NVP), methacrylic acid (MAA), methyl methacrylate (MMA), and polyvinyl alcohol (PVA), aiming to improve hydrophilicity of silicone or silicone hydrogel-based contact lenses while maintaining their high oxygen permeability. However, the plasma treatment involves high production costs and complicated manufacturing processes. The conjugation of hydrophilic materials on surfaces of silicone or silicone hydrogel-based contact lenses is highly unstable, leading to high defect rate and unreducible production costs.
Another method for production of silicone hydrogel-based contact lenses takes the respective advantages of hydrophobic and hydrophilic materials and conjugates hydrophilic biomedical materials, such as HEMA, GMA, GMMA, NVP, MAA, MMA, and PVA, with silicone and hydrogel prepolymers, mainly hydrophobic polysiloxane or fluorinated silicone with high oxygen permeability, to improve water retention and wearability of silicone contact lenses.
However, quality control during such production method has been difficult due to its complicated manufacturing process and high production cost of the prepolymers. Additionally, while such method increases hydrophilicity of contact lenses, surface tension of silicone prepolymers hinders the distribution of hydrophilic materials on the lens surface, yet causing multiple hydrophobic dry spots. Moreover, phase separation is also a common issue and would result in opaque lenses with reduced light intensity. Consequently, discomfort sensations while wearing contact lenses have yet remained a challenge in the field.